Combine water, yeast, and sugar in a bowl and let bubble for 5 minutes to activate the yeast
Make a tangzhong by heating 15g (2 tbsp) flour and all the milk while stirring constantly until thick, about 1-2 minutes
Mix the flour, corn starch, baking powder, salt, bloomed yeast, and tangzhong together into a dough
Knead until loosely together
Add butter and knead until smooth
Let rest covered for 1-2 hours
Knead out all the air and roll into a cord
Divide into ping-pong-sized balls
Roll into disks with thin edges and slightly thicker centers
Add filling and let rest for at least 20 more minutes before steaming, spritz with water to prevent skin
Steam for 20 minutes, then rest for 10 minutes before removing the lid
Cool to room temperature with lid on before freezing if desired
